Part Pig, All Human

It's one thing to be a guinea pig for medicine, but in 1996, Jim Finn agreed to be part pig.

Finn was a victim of Parkinson's disease and allowed surgeons to inject his brain with 12 million brain cells from a pig. Researchers hoped that the fetal pig cells might replace functions he lost from the disease.

"This gave me my life back," said Finn. "By now, I'd be unable to drive a car, unable to walk. Now I can walk."

Finn's improvements made him the self-described poster child of a science called xenotransplantation – a process that uses animal cells or organs to treat human disease. Despite Finn's progress, the process that saved his life can put others in danger from a virus.

Some are worried that xenotransplantation allows viruses to jump within species, reports CBS News Correspondent Wyatt Andrews. Dr. Jonathan Allan, an expert in the transmission of viruses, said, "My major concern in that a pig virus may find its way into a human recipient."

"The most important danger is the risk of a new epidemic," said Allan.

In the lab, pig cells are known to transfer "pervs," porcine endogenous retroviruses, into human cells. But, unlike monkeys, no pig retrovirus has ever made a human sick. It gets in, but seems to go nowhere.

"I don't know what to expect. The perv virus may do nothing, but these are questions that need to be resolved," cautioned Allan.

The risk of pervs, however small, makes Jim Finn a guinea pig for life. Doctors must routinely check him and more than 100 other pig-cell-patients worldwide for infections. So far, none have contracted a virus to be alarmed about.

Some people are worried that patients like Finn could be carriers of sleeper viruses that could infect the rest of the population. "That's the possibility. It can't be denied," responded Finn.

But what can not be denied, is that he is walking – over five years later.

"Research has got to continue. There are too many people sick and need these cells and organs and tissues," said Finn.